Vacuum pumps typically comprise an impeller in the form of a rotor mounted on a shaft for rotation relative to a surrounding stator. The shaft is supported by a bearing arrangement comprising two bearings located at or intermediate respective ends of the shaft. One or both of these bearings may be in the form of rolling bearings. Usually, the upper bearing is in the form of a magnetic bearing, and the lower bearing is in the form of a rolling bearing.
A typical rolling bearing comprises an inner race fixed relative to the shaft, an outer race, and, located between the races, a plurality of rolling elements for allowing relative rotation of the inner race and the outer race. To prevent mutual contacts between the rolling elements, they are often guided and evenly spaced by a cage. Adequate lubrication is essential to ensure accurate and reliable operations of rolling bearings. The main purpose of the lubricant is to establish a load-carrying film separating the bearing components in rolling and sliding contact in order to minimise friction and wear. Other purposes include the prevention of oxidation or corrosion of the bearing components, the formation of a barrier to contaminants, and the transfer of heat away from the bearing components. The lubricant is generally in the form of either oil or grease (a mixture of oil and a thickening agent).
Vacuum pumps using oil-lubricated bearings require an oil feeding system for feeding oil between the contact areas of the bearing, which enables the oil to perform cooling as well as lubrication and thereby permit the bearings to run at a faster speed. Turbo-molecular pumps have traditionally used a wicking system for supplying oil to a rolling bearing. In such a system, a felt wick partially submerged in an oil reservoir feeds oil to a conical “oil feed” nut mounted on the shaft. With rotation of the pump, oil travels along the conical surface of the nut to the bearing. The oil passes through the bearing and is returned to the reservoir.
Felt is formed by matting a large number of individual filaments, and so the felt wick is normally treated with a rubber stabilising agent. This is because untreated felt wicks are prone to release debris, usually in the form of individual filaments, parts of filaments, or dirt trapped within the filaments during matting, into the oil. This debris is subsequently carried by the oil into the bearing. The resulting contamination of the bearing can cause damage which promotes premature bearing failure. Whilst washing the wicks can reduce the amount of dirt-related contamination of bearings, the washing process tends to damage the integrity of the felt, leading to increased release of filaments and filament fragments during use.
There are also problems associated with the use of treated fibre wicks. The stabilising agent used to treat the wicks is temperature sensitive. The high relative velocities (typically in excess of 25 ms−1) experienced within a turbomolecular vacuum pump cause significant quantities of frictional heat to be generated. With prolonged use, the treated wicks can become deformed and hardened as the tip of the wick begins to char and consolidate. This hardening can significantly reduce the rate at which oil is transferred from the reservoir to the bearing as the consolidation of the felt and the deformation of the tip reduces the ability of the wick to retain good contact with the surface of the nut. This reduction in the rate of transfer of oil causes further charring and deterioration of the felt wick. Consequently, premature bearing failure can occur through wear damage and overheating caused by poor lubrication.
Conventional materials used for manufacturing bearing cages, such as Phenolic resin, tended to absorb some of the oil. This oil retention caused the bearing to be reasonably tolerant to a non-lubricated environment as the cage material was able to act as a further source of lubricant. More recently the cages of bearings are being manufactured from a single piece of material which, whilst being more accurate and better balanced, is more sensitive to wear. In particular the new materials being used, e.g. Torlon®, do not absorb oil in the same manner as the conventional materials, and therefore bearings with cages manufactured from these new materials are much less tolerant of lubrication starvation.